| Year | Number of sites | Total area (ha) |
|---|---|---|
| 1990 | 28 | 302000 |
| 1995 | 28 | 302000 |
| 2000 | 28 | 303000 |
| 2001 | 28 | 303000 |
| 2002 | 28 | 301000 |
| 2003 | 28 | 301000 |
| 2004 | 28 | 313000 |
| 2005 | 28 | 313000 |
| 2006 | 28 | 314000 |
| 2007 | 30 | 291000 |
| 2008 | 30 | 291000 |
| 2009 | 33 | 373000 |
| 2010 | 34 | 360000 |
| 2011 | 33 | 368000 |
| 2012 | 265 | 368000 |
| 2013 | 266 | 384000 |
| 2014 | 266 | 384000 |
| 2015 | 266 | 383200 |
| 2016 | 266 | 383100 |
| 2017 | 266 | 383100 |